A person's ability to maintain body position and attitude, whether engaged in a relatively stationary activity such as standing or in a dynamic activity such as walking or running, is dependent on the person's balance control. Balance control is often considered to distinguish two different types of balance control: anticipatory balance control; and compensatory balance control.
Anticipatory balance control provides anticipatory postural adjustments (APAs) of attitude and position of body parts that are undertaken by a person in response to activities involving anticipated postural challenges. For example, walking up a step, which entails balancing the body on one foot in order to step up, is an activity that involves an anticipated postural challenge that elicits anticipatory balance control. Stepping onto a moving walkway in an airport, which entails preparing to maintain stable upright posture during a relatively fast transition from a stationary to a moving walking surface, is (usually) an anticipated postural challenge that elicits anticipatory balance control.
Though people do not normally think of anticipatory postural adjustments as voluntary, they are considered to be voluntary activities of the nervous-muscular system because they involve decisions, volitional decisions, which are made by the brain. Anticipatory postural adjustments are usually performed in periods of time between about 150 to 200 milliseconds (ms).
Compensatory balance control provides compensatory postural adjustments of attitude and position of body parts in response to, and to recover from, unanticipated postural challenges. For example, slipping on a wet spot on a marble floor, or catching ones foot on a curbstone, are typically unanticipated postural challenges that elicit compensatory balance control to provide compensatory postural adjustments in order to prevent falling. A sudden braking or lurching of a subway car is an unanticipated postural challenge to a person standing in the subway car that elicits the person's compensatory balance control to provide compensatory postural adjustments to prevent falling.
Compensatory postural adjustments (CPAs) are considered involuntary, reflex postural adjustments, because, while orchestrating complicated muscle synergies, they are not under volitional control and do not directly involve decisions by the brain. Compensatory postural adjustments are typically faster than APAs and are usually performed in periods of time less than about 100 ms.
Anticipated balance control, and/or compensatory balance control, often degrades with age and/or injury to the nervous and/or muscular system. In the elderly, degradation of balance control results in a relatively high rate of serious and fatal injuries. In the United States and in Israel, on the average one out of every three elderly people over the age of 75 years old and 50% of those who are 80 years old and over falls every In 2000, the total direct cost of all fall injuries for people 65 and older exceeded $19 billion. The financial toll for older adult falls is expected to increase as the population ages, and may reach $54.9 billion by 2020 (adjusted to 2007 dollars). In a study of people age 72 and older, the average health care cost of a fall injury totaled $19,440, which included hospital, nursing home, emergency room, and home health care, but not doctors' services.